1. Field of the Invention
The present invention relates to the field of refrigerant, compressed gas/air dryer systems, and more particular to a refrigerant ‘feed’ process employing multiple staged expansion valve injection inputs affording significant system stability and effectiveness.
2. Background
Presently, many industrial applications using air or gas driven machinery have a need for dry air or gas in the process of operating, product process, product fabrication, as well as many other applications. Air or gas driven machinery is most commonly operated using pressurized, i.e., compressed air or gas that contains water that can react on or condense within product or apparatus and negatively impact the air or gas usefulness. When compressed the partial pressure of water will increase as the volume decreases. Moisture in the form of condensation or precipitation in machinery or on product negatively impacts the product process systems by causing costly equipment maintenance or equipment failure and befouled product.
Refrigerant dryers are the most common devices to remove moisture from compressed air or gas for such industrial uses, thus reducing failures and improving product quality. The water content quality of the air or gas being dried, at the dryer's output is measured in terms of dew point, the temperature where water vapor in the air or gas is at 100% humidity; the lower the dew point temperature, the greater the dryness of the air or gas. Dryer air or gas is considered higher quality. Industry desires air or gas of sufficient quality to prevent water from damaging machinery or fouling product.
There are several types of refrigerant air or gas dryers, the following list includes more conventional systems: Cycling Dryers, Non-Cycling Dryers and Variable Speed Drive Dryers. In general, refrigerant air or gas dryers have: 1) a refrigerant compressor (with an appropriate accumulator and receiver); 2) a series of heat exchanger vessels and/or other ‘heat’ transfer components; 3) a condensing component; and a 4) a refrigerant process controller having one or more of the following: expansion, pressure regulating, bypass valves; solenoids and electronic sensors/controls; an optional variable speed drive (VSD) system for the compressor motor.
These systems all operate on various levels of efficiency, both with respect to cost and dew point performance. In common practice, a cycling air or gas dryer includes an unloading feature that to allows the compressor motor to power down, i.e., to coast or free wheel during periods of low demand for refrigerant cooling. Thus, a cycling dryer is considered an energy savings dryer when compared to a conventional non-cycling system. Another example of energy savings may be found in a system configured with a variable speed drive (VSD) device to decrease power to or to slow-down the compressor during lull intervals, periods of less demand for refrigerant cooling. Such a system is also considered to be an energy savings dryer because the compressor consumes less energy during the lull intervals.
However, each of these above listed devices use an expansion valve to feed or deliver compressed refrigerant into a heat exchange type vessel, where the expansion of refrigerant produces a coldest point for heat exchange purposes. Expansion valves rely on a temperature and pressure feedback which causes the valve opening to either increase in size for greater refrigerant feed, or, decrease in size for less refrigerant feed. These valves are conventionally available as strictly mechanical valves or in combinations of mechanical and electrical and/or electronic (solenoid, proportional, step motor drive, etc. with or without microprocessor control) valves; all having the desired goal to feed expanded refrigerant as required by the recycling means back to the refrigerant side of a heat exchanger providing a coldest point for thermal exchange.
One of the problems in this type of device is that the expansion valves are called-out in terms of tonnage (the capacity with which the device can deliver expanded refrigerant and feed the system). The tonnage is expressed as a range based on differential pressure; for example, 10 tons (generally for operating in systems from about 80,000 btus to about 120,000 btus capacity requirements). When these devices are specified in the design of a system, the tonnage expressed could actually be implemented as on the low side, in the mid range, or, on the high side of the valve capability to deliver refrigerant feed. This means, in simple terms, that the valve in any particular system may be required to work near maximum capacity, in a mid range, or barely working efficiently at low capacity, each, respectively in each design. That equates, in each of the scenarios, to the valve working less than ideal for most of the range of the system designs capacities.
To broaden the range of efficient operation of the valves used in varied systems, the expansion valve is conventionally adjusted. The adjustment is expressed in terms of superheat; a value derivative equivalent to the refrigerant systems compressor suction pressure converted to degrees in temperature (as related to a specific refrigerant type) and subtracted from the refrigerant systems suction temperature.
An expansion valve may be generally used over a wide tonnage range. Thus factory adjustment for superheat is undesired. Each valve must be set for superheat to reflect 10-15° F. over room temperature. To set the superheat, one must use a thermocouple or thermometer to measure the temperature of the suction line, for example, at a thermal bulb. Then one measures the pressure in the suction line at the thermal bulb well or external equalizer. The measured suction is then converted to a pressure equivalent saturated temperature using a pressure temperature chart. The difference between this value and the temperature measured at the thermal bulb well is expressed as the superheat. Superheat is often in the approximate range of five to ten degrees F.
Ideally, the superheat (a value derived from suction temperature and suction pressure), gives feedback to the expansion valve to close-down (a call for less refrigerant) to maintain a predetermined level of performance. Conversely, when the call is to increase refrigeration, the change in superheat causes the expansion valve to open-up and feed more expanded refrigerant.
Unfortunately, no valve devices work at an optimum under most or all conditions in any given system or design. In practice the parameters routinely overshoot. The result is an expansion valve hunting endlessly. That is the valve will open-up for more feed which will be followed by a close-down because of too much feed, and again, an open-up because of too little feed resulting in a drop of the flooding level; resulting in a never ending cycle. This phenomenon occurs in every system at some point even in carefully designed systems using the mid range as ideal or with sophisticated electronically assisted expansion valve devices. This hunting, over/under, constant pursuit to satisfy the endless loop of superheat feedback results in less than ideal performance of the gas/air dryer system desired to produce a low, constant dew point temperature gas or air. The hunting results from the refrigerant being returned in an erratic manner.
Another problem with conventional refrigerated compressed air dryers systems is when the refrigerant causes ‘freeze-up’ of the heat exchanger system because the expansion valve is opened too much or for too long a period of time and conversely, when the expansion valve opening is closed too much or for too long, the gas/air dryer system would suffer poor performance with respect to dew point.
Various patented devices have been designed to overcome poor performance with respect to dew point. U.S. Pat. No. 6,516,626 (Escobar) discloses a two stage refrigeration system incorporating a means for storing refrigerant vapor and slurry having a receiving tank or tanks. U.S. Pat. No. 6,490,877 (Bash) teaches parallel evaporators and a means to control the mass flow rate of the refrigerant to each evaporator. U.S. Reissue Pat. No. RE 33,775 (Behr) teaches the use of multiple evaporators and method of controlling the valve in a refrigeration system. However, the various systems are undesirable in that they do not provide a means for staged feed injection of to deliver refrigerant to a single evaporator system and process to maintain stable, balanced parameters affording a very high performance in dew point of a gas/air dryer system. These inventions suffer from the fact that they do not provide for smaller capacity adjusted to the ‘higher’ end of their ranges while the larger capacity is set to their ‘lower’ end of adjustment process. Also, these inventions do not adequately track the demand for refrigerant and thus fail to modestly modulate the valve to result in perfect output of dew point temperature according to the gas/air dryer's capacity.
Thus the state of the art is clearly not ideal. Normal load changes during industrial cycles can adversely affect dryer operations, resulting in poor dew point performance, waste of energy and wear-and-tear of equipment. The industry has accepted that it is the nature of refrigerated gas/air dryer systems (even those having sophisticated electronically assisted expansion valves) to function with cyclical operation expansion and thus routinely experience the same over/under performance.
Thus it is readily apparent that there is a longfelt need for structure and process such as a plurality of refrigerant expansion valves where “staged” feed is injected into the refrigerant side of the evaporator heat exchanger to affect a more controlled means to ‘deliver’ the expanded refrigerant into the heat exchange to maintain stable, balanced parameters affording a very high performance in dew point of a gas/air dryer system.